Can't recompile decompiled class files

Question

I have a bit of Java I want to edit (it is a Minecraft mod) I decompiled the .class files (with jd-gui), edited what I wanted to, then attempted to recompile (with javac).

I got hundreds of errors. I only edited a few lines though, so I tried compiling the .java's that were unedited, exactly as they were from the decompiler. Same thing, hundreds of errors.

Know very little about Java, so I am certain I am making some mistake. Why can't it compile code that has not been edited from a valid working .class file?

Here is a sample of what I get:

ReiMinimap.java:451: illegal start of expression
              switch (???)
              ^
ReiMinimap.java:451: ';' expected
              switch (???)
                    ^
ReiMinimap.java:451: illegal start of expression
              switch (???)
                      ^
ReiMinimap.java:451: illegal start of expression
              switch (???)
                        ^
ReiMinimap.java:451: illegal start of expression
              switch (???)
                         ^
ReiMinimap.java:452: illegal start of expression
              {
              ^
ReiMinimap.java:452: : expected
              {
               ^
ReiMinimap.java:453: ';' expected
              case 49:
                     ^
ReiMinimap.java:453: ')' expected
              case 49:
                      ^
ReiMinimap.java:454: illegal start of expression
                this.allowCavemap = true;
                    ^
ReiMinimap.java:454: ';' expected
                this.allowCavemap = true;
                     ^
ReiMinimap.java:454: illegal start of expression
                this.allowCavemap = true;
                                  ^
ReiMinimap.java:454: ';' expected
                this.allowCavemap = true;
                                   ^
ReiMinimap.java:456: orphaned case
              case 50:
              ^
ReiMinimap.java:308: 'try' without 'catch' or 'finally'
    try { if (paramMinecraft == null);
    ^
ReiMinimap.java:499: illegal start of type
      else
      ^
ReiMinimap.java:499: ';' expected
      else
          ^
ReiMinimap.java:501: illegal start of type
        this.chatWelcomed = true;
        ^
ReiMinimap.java:501: <identifier> expected
        this.chatWelcomed = true;
            ^
ReiMinimap.java:501: ';' expected
        this.chatWelcomed = true;
             ^
ReiMinimap.java:501: illegal start of type
        this.chatWelcomed = true;
                          ^
ReiMinimap.java:501: <identifier> expected
        this.chatWelcomed = true;
                           ^

And some of the code:

if ((!this.chatWelcomed) && (System.currentTimeMillis() < this.chatTime + 10000L))
      {
        Object localObject1;
        for (localObject2 = this.chatLineList.iterator(); ((Iterator)localObject2).hasNext(); ) { localObject1 = (ahe)((Iterator)localObject2).next();

          if ((localObject1 == null) || (this.chatLineLast == localObject1)) break;
          Matcher localMatcher1 = Pattern.compile("§0§0((?:§[1-9a-d])+)§e§f").matcher(((ahe)localObject1).a);
          while (localMatcher1.find())
          {
            this.chatWelcomed = true;
            for (??? : localMatcher1.group(1).toCharArray())
            {
              switch (???)
              {
              case 49:
                this.allowCavemap = true;
                break;
              case 50:
                this.allowEntityPlayer = true;
                break;
              case 51:
                this.allowEntityAnimal = true;
                break;
              case 52:
                this.allowEntityMob = true;
                break;
              case 53:
                this.allowEntitySlime = true;
                break;
              case 54:
                this.allowEntitySquid = true;
                break;
              case 55:
                this.allowEntityLiving = true;
              }
            }
          }

did you add the original library to the classpath when you compile? — Denis Tulskiy, Nov 21 '11 at 06:21
As far as examples, do you just want me to copy the entire error log? Also: original library? I know basics of programing, and other unrelated languages, but almost nothing about Java. Not sure what you mean. Which could explain why I have an issue. . . — zeel, Nov 21 '11 at 18:50
@zeel: Not necessarily. Just show an example of the kind of error and the code that causes it. We have no idea what's on your screen so it's impossible to properly diagnose it otherwise. — Boann, Nov 21 '11 at 21:53
@zeel: You still didn't show the actual code. But "switch (???)" is no kind of valid Java. Get a better decompiler. — Boann, Nov 22 '11 at 21:01
Oh you wanted the code! okay. Also, any decompiler recommendations? (added some of the code) — zeel, Nov 22 '11 at 21:08

score 2 · Accepted Answer · answered Nov 21 '11 at 04:48

2

It sounds like the original Java compilation was obfuscated. There are many tricks that obfuscators use to make it difficult to reverse engineer code. For instance, it is legal to have method and field names in .class files that are reserved words in Java source (for, if, etc.) If the obfuscator uses such tricks, then when you decompile the .class file, the Java source is illegal because you can't use those reserved words as method or field names.

answered Nov 21 '11 at 04:48

Ted Hopp

232,168
48
399
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Indeed, that's what I've found as well, obfuscators are designed precisely to make this kind of reverse-engineering more difficult. You might actually have more luck modifying the underlying byte code than trying to recompile the decompiled source code. – Ewald Nov 21 '11 at 05:48
1

None of the code looks obfuscated, I have seen obfuscated code, this doesn't look like it. – zeel Nov 21 '11 at 11:19

Can't recompile decompiled class files

1 Answers1